1. Field of the Invention
The present invention relates to an improved steering apparatus for a vehicle having steerable front and rear wheels.
2. Description of Relevant Art
There has been proposed a four-wheel steering apparatus of the so-called symmetric function type in which the maximum steering angles of the rear wheels, when the rear and front wheels are turned in one direction and opposite directions, are equal to each other. Such a four-wheel steering apparatus is disclosed in Japanese Laid-Open Patent Publication No. 58-209657, for example. The disclosed steering apparatus includes a rear wheel steering system having an input shaft rotatable in response to the rotation of a steering wheel, an eccentric shaft mounted on the rear end of the input shaft, a joint member rotatably fitted over the eccentric shaft and interconnecting the respective tie rods of the rear wheels, and a link mechanism by which the joint member is supported on a vehicle chassis for preventing the joint member from swinging.
Another steering apparatus is diclosed in earlier Japanese Laid-Open Patent Publication No. 58-30869. This prior steering apparatus differs from the steering apparatus disclosed in the aforesaid publication in that the eccentric shaft and the joint member are held in slidable engagement with each other to cause the joint member to be moved only in lateral directions.
The steering apparatus of Japanese Laid-Open Patent Publication No. 58-209657 suffers the following drawbacks: Since the link mechanism is employed to prevent the joint member from swinging, a space is required to accommodate such link mechanism, with the result that the steering apparatus cannot be reduced in size. It is difficult to confirm the performance of the steering apparatus before it is installed in the vehicle. It is also difficult to achieve a desired degree of mechanical strength and accuracy of the link mechanism as attached to the vehicle chassis. Since the link mechanism extends laterally toward one chassis side only, the resulting structure is asymmetrical, whereby the ball-and-socket joints by which the tie rods are coupled to the joint member are displaced in slightly different manners when the input shaft is rotated in opposite directions. This has resulted in an error or difference in the angles through which the rear wheels are steered.
Specifically, as the joint member is subject to combined vertical and lateral motions the ball-and-socket joints are also moved vertically causing the rear wheels to be steered through different angles. Finally, the input shaft is supported by a relatively thin holder which is subjected to an intensive force and hence cannot be supported on a resilient rubber mount. Therefore, difficulty has been experienced in insulating noises and vibrations effectively.
In the steering apparatus disclosed in Japanese Laid-Open Publication No. 58-30869, a major drawback results from the fact that the eccentric shaft which is directly fitted in the joint member must necessarily be centered with respect thereto with high accuracy, whereas the presence of a radial clearance is unavoidable between the eccentric shaft and the joint member.